3D Print a solid object
Well, that probably means that you need to make the coin thicker. With the coin selected, tab into edit more. Press A to select all. Then press S to scale them. You might also want to make the coin's features (numbers, etc.) bigger as some fine details cannot be printed depending on the extruder size that company uses. 

This issue is definitely not caused by the object being a hollow shell. As long as the mesh is closed, slicers will recognize it as a solid object. As a 3D printer owner, I can easily verify that.